With age a large percentage of the population develops atherosclerotic arterial obstructions resulting in a diminished blood circulation and a variety of related disorders. Presently such obstructions are circumvented by surgically grafting a bypass or they are treated by a catheter equipped with a balloon which is inserted through the arterial system, over a flexible guide-wire, into the obstruction and then inflated to expand the obstruction's lumen (Angioplasty). Some of the problems with Angioplasty are that it injures the arterial wall, it creates a rough lumen and in substantial number of the cases it is ineffective. Further, Angioplasty does not capture and remove the obstruction material out of the arterial system, therefor Angioplasty carries the risk of dislodging obstruction material and allowing it to move down stream creating additional blockages.
An objective of the present invention is to provide an over a guide-wire rotary Atherectomy catheter system, which cuts and removes the obstruction material and which is equipped with torque limiting clutch means to prevent damage to the arterial system. The system may be introduced into the arterial sytem at the groin area, through a sheath to reach a work site in the coronary arteries, which requires a long catheter with an elastic, supple, distal section, a stable diametrical cross section and a torque transmitting ability through the various arteries it is passed through. Meeting these design objectives simultaneously, limits the strength of the distal section (as shown in FIG. 1, the atherectomy system comprises several elongated parts in a nested relationship, and their ends or sections shall be referred to as "distal" meaning the end which goes into the vessel and "proximal" meaning the other end, thus, "distal direction" or "distally" shall indicate a general direction from the proximal end to the distal end, and "proximal direction" or "proximally" shall refer to an opposite direction).
The rotary cutting action which takes place at the distal end of the flexible rotary-catheter requires a certain amount of net torque and rotation. To provide it, the flexible rotary-catheter is driven at its proximal end by a motor, however, due to frictional losses along the length of the flexible rotary-catheter the gross torque that is required at the proximal end is substantially larger than the net torque. To withstand the gross torque the proximal section is strengthened, for example, by increasing its wall thickness or by reinforcing it, and since the proximal section is often disposed in relatively straight arteries, its increased stiffness is acceptable. However, due to various unpredictable and uncontrollable variations that may occur during an individual procedure, such as changes in coefficient of friction between the rotating parts of the system and the stationary parts of the system or of the artery, or changes in the forces inducing the friction which may occur due to misalignment of parts of the system, or a spasm in the artery which may lock the artery onto the rotary catheter, the pattern of torque distribution along the frc may be altered, and it is an important objective of this invention, even under such circumstances, to protect the arteries against injury by either over torquing the artery directly or by over torquing and fracturing the flexible rotary-catheter which may in turn cause injury to the artery.
A numerical example may be helpful: Assuming that the required net and gross torques are 1 inch-ounce and 5 inch-ounce, respectively, and that the ultimate strength of the distal and proximal sections is 3 inch-ounce and 9 inch-ounce, respectively, and further assuming that the motor drive can deliver 7 inch-ounce at stall but because of inherent torque/speed characteristics it will deliver as much as 12 inch-ounce while turning. Thus, it can be seen that if due to an arterial spasm or other cause the blade became locked in the artery, even momentary, the motor may deliver close to 12 inch-ounce of torque to the blade (as previously discussed the frictional losses along the flexible rotary-catheter may occasionally be very small) fracturing the distal section, causing serious complications. If the spasm occurs at a point along the proximal section it can have the same undesirable results. To prevent such complications a first torque limiting clutch rated at 2 inch-ounce is interposed between the distal section and the proximal section and a second torque limiting clutch rated at 6 inch-ounce is interposed between the motor and the drive's output shaft. The first torque limiting clutch enables sufficient working torque to be transmitted to the distal section but prevents destructive torque from reaching it; the second torque limiting clutch does the same for the proximal section.